Receiving system for frequency or pulse modulated electromagnetic waves



July 26, 1949.

EARP RECEIVING SYSTEM FOR FREQUENCY OR PULSE Filed June 12, 1943 MODULATED ELECTROMAGNETIC WAVES l/V TERMED/A r5 FREdl/E/Vt') VAR/A BL E BAA/0 AMPLIFIER F/G 2 Sheets-Sheet 1 FREQUL'NCY CHANGER.

W TM HIGH FREQUENCY AMPLIFIER. I D

RESPONE (AR/THMET/C SCALE) 1 July 46, 1949. c. w. EARP 2,476,964

RECEIVING SYSTEM FOR FREQUENCY 0R PULSE V MODULATED ELECTROMAGNETIC WAVES Filed June 12, 1945 2 Sheets-Sheet 2 Rssouss Patented July 26, 1949 RECEIVING SYSTEM FOR :FREQUENGIJ' QR MODULATED ELEGTROMAGNETIG Charles William Earp, London, England, assignor,

by zmesne assignments, to International htand- 2rd Electric Corporation, New York, :N. Y}, .a

corporation of Delaware lowing.

Application June 12, 1943, "Serial No. 49.11576 In Great Britain July 11'', 1942 7 Claims.

:1 The present invention relates to 'a method of and arrangements for the reception of electromagnetic waves modulated in such manner .as to vary the transmission :frequency :band width with depth of modulation.

vary over this range of modulation.

limits.

modulation.

lation.

2 tion. For example, t! *the'ptilse train is timeanodmated, that is, the time interval between pulses is varied 'or modulated, or in other words, the repetition frequency or the pulse @groupqs'hase The invention reis modulated, the total transmission is a mulslates particularly, though not specifically, to a tiple frequency modulated transmission. Simimethod of and arrangements for the reception 'larly, a modulation of the high frequency phase of frequency modulated (electromagnetic waves of successive pulses causes phase modulation :of or .a train of modulated electrical pulses. all the separate rcarrier waves which constitute is the (object ofthis invention Ito provide the puls train. The expression "depth oi modarrangements of the type -specified ahigh signal ulation used in the appended claims is intendto noise ratio 'for both high and low levels of ed to mean the amount of "shift of the carrier modulation. 'The theoretical possibility of such frequency .for frequency modulation and the arrangements is evident from the fact :that 'over :amount of change "in the repetition frequency or .a wide range of modulation of such transmisthe phase shift for pulse modulation. sions, the s'i'de'band content is substantially c0n- The ideal receiver for the case of .a frequency stant, so that signal to noise ratio should not modulated transmission will have -'a single passband which expands and contracts with trans- The advantages of arrangements as provided mitter modulation depth. by the present invention are most apparent The ideal receiver for pulse modulation syswhere it is desired to transmit :an intelligence tems "will have "a plurality of pass-ebands, each wave which varies in amplitude hetween wide of which expands and contracts according to For example, in order to transmit orthe depth of modulation. This latter receiver 'chestral music, a signal to noise ratio 80 db. for may rbe provided 'a comb-like filter i. e. :a peaks of modulation must normally be obtained filter having fa comlza-eshalr d F P 861 2611 3 in order that a reasonably good signal to noise characteristic, in which the response teeth ratio can be obtained during the troughs of have a variable thickness-or hand-width In the arrangements to :be defilters are more fully described hereinafter. scribed, the signal to noise ratio can be main- According to the most general inspect of the tained at say, db. in the'tr'oughs of modulation, 30 present invention, the method of reception of while not requiring the high power of transmiselectromagnetic waves modulated in such mansion associated with a, peak signal to'noise 'ratio nor as to vary the transmission freq en y andwidth with depth of modulation comprises vary- The principle of this invention :lies in the :folin'g automatically the receiver operatin band- The frequency band-width occupied,' width accordin to the depth of modulation of and the resultant .noise power which must be .rethe received waves. ceived, varies according to the amount of modu- Arrangements for carrying out the method :of The receiver band-width is, therefore reception acco ding to t e invention comprise automatically adjusted according :to the :bandmeans for producing a, voltage proportional t widthoccupied'by the transmission. 40 the depth of modulation in t received waves In the case of a frequency modulated wave, and for applying said voltage to control autothe transmission frequency spectrum occupies a matically the operating frequency band-width of frequency band w'hichexpands and contracts "acthe receiver. In a practical arrangement, a cordin to the amount of modulation. variable hand-width filt r is provided and takes In the case of pulse trains in which for zero the form of an ampl fier wi h ne ative and imemodulation the repetition frequency is constant itive feedback paths. at I per second, the unmodulated transmis- The invention will be better understood after =s'ion can 'be considered as a spectrum of carrier reading the following description given in conwaves which are spaced at equal intervals of f junction with the accompany'ing drawings which cycles per second over the frequency band. show diagrammatically the circuit arrangements Modulation of such a pulse train produces modof one practical embodiment of the invention. 'ulation of these carrier waves, and causes the in'the-drawings, transmission to occupy a large number of sep- Figure 1 is a block schematic diagram ol the arate frequency hands, all of'whieh expand and general receiving arrangement embodying th contract according to the amount of modul'ao5 invention.

2,476,964 I -3 f I.

proximately as shown in Fig. 3. Rectifier R produces a D. C. voltage across R2 which voltage is approximately proportional to the frequency deviation of the; received signal; 7

Fig.4 shows a modified arrangement which gives a better overall characteristic. In this, two tuned circuits L505 and LsCs are used, one tuned to slightly higher than carrier frequency, and one slightly lower and feeding separate rectifiers R5R6 network via condensers C'zCs.

teristic curve of the filter shown in Figure 5.

Figures 7 and 8 show diagrammatically circuit arrangements for variable frequency band filters suitable for incorporation in the arrangements shown in Figure 1 and for the reception of frequency modulated waves and trains of modulated pulses respectively. 1

- Figure 1 shows a block schematic diagram of a suitable receiver for carrying out the invention.

. In this diagram-,- only the more essential parts are shown, in order to show the application of the invention to both frequency modulated and-time modulated: pulse systems. In thecase of frequency modulation'the usual limiter stage would, of course," be added before the demodulator.

Similarly, a modulated .pulse system receiver Znight include trigger circuits for noise elimina- After optlonalhigh frequency amplification in H. F. and frequency changing in F0, the signal is amplified in IFA. Here, the signal is fed into two paths. In the first path, the signal passes via a .delaylink DL, or path of appreciable transit time. This delay link DL may be comprised of a number of tuned circuits coupled together to form a band pass filter. I

From DL, the signal passes through the variable band-width filter VF. This variable filter may have .a single, or multiple response bands as hereinafter more fully described, which are :variable in width. cAfter filtering, the signal is demodulated by known means in D; which may, for example, be a frequency discriminator.

This demodulator yields the low frequency modulating wave, which may be raised to the required level in the amplifier LFA.

The secondpath-of the signal from IFA is to th slot filter SF. This is a filter which passes little or no signal when the latter is unmodulated,but which gives a response rising with the amount of modulation. Suitable slot filters" are described hereinafter.

The wave which passes through theslot filter is rectified and detected in rectifier R yielding a direct-current which is zero for no modulation, but which rises in level according to the amount of modulation.

This direct current is used to control the band-width of the variable filter VF. For small depths of modulation the band-width of VF is small, but when anegative bias is applied to it by rectifier R, the pass bandexpands,

:wave, Sidebands are approximately tuned in L402 to yield an overall filter characteristic apwhich in turn feed a common load resistor R2. The tuned circuits are fed from the bridged-T The voltage produced across- R2 is then applied to control the variable filter VF (Fig. 1). V

I Figure 5 shows a multi-slot or comb-like filter circuit especially suitable for the case of a pulse transmission. In this, the signal is passed equally through two parallel paths, one path including a delay link DLI. of delay equal to the pulse repetition period, and the other including an aperiodic pad P which gives an attenuation equal to that of the delay link DLI. The multiple slot filter has a response frequency characteristic as shown in Figure 6, rejecting all frequencies which appear in the unmodulated pulse transmission. Whilst adelay'network is shown-in one path, it will-be understood that'a differential delay equal to the pulse repetition period between the two 7 paths should beiobtained in practice.

The output of the filter shown in Figure 5 is fed to a rectifying unit as in Figure 2 or 4.

Suitable variable-bandwidth filters vF' f V Figure 7-shows the schematic diagram of a filter. which provides a single'passband controllable by means :of grid-bias to valves comprised 'in the filter. This filter is in principle an. amplifier designed to pass the maximum'band which may be occupied by the signal, with'the following additions: a

1. The amplifier is provided with a large amount of aperiodic reverse feedback, by means of cathode resistances R10 and R20.

2.'The amplifier is provided with selective positive feedback by the coupling links CLl' and GL2 between'TC l to T02 and TC5 ,to'TC3 respectively.

Grid bias control for the valves'is supplied from the output of R (Figurel) as described and.indicated at BC in Figure '7.

Forsmall'values of grid-bias, the amoimts of positive and negativefeedback are adjusted, by adjusting R10 and Ra io give a single but stable narrow response band. Application of bias, of course, removes feedback causing the filteramplifier .to give a broader response up to the.

maximum bandwidth as determined by the tuned circuitsv TCl-T C6 without feedback.

Figure 8 shows one form of variable filter;especially suitable for a pulse transmission; This variable filter is essentially a broad-band amplifier in which a considerable amount of aperiodic reversefeedback is applied, plus a-further'feedback loop 'containi'nga delay network DNZ.

Owing to the rotation of phase with frequency in the delay network DN 2,-this feedback loop be received. I

Under the control of bias obtained from R (Figure 1) and applied to BC, which cuts down valve gain and hence removes feedback, the filter-amplifier pass bands may be expanded until, if required, no gaps are left in the overall response curve.

In the filter amplifier of Figure 8, the reverse feedback is applied separately on each stage of the amplifier by using inverted amplifiers; that is, grids are grounded, and inputs are applied to the cathode. The delay network DN2 is comprised of a number of tuned circuits coupled together to provide a broad pass-band, and a transit time or delay equal to the pulse repetition period.

What is claimed is:

1. Receiving apparatus for the reception of frequency modulated electromagnetic waves and having a single variable frequency bandpass control comprising an amplifier having positive and negative feedback paths in each stage, means for producing a voltage responsive to the depth of modulation of the received waves, and means to apply said voltage to vary automatically the amount of the negative feedback in said amplifier.

2. Receiving apparatus, as claimed in claim 1, wherein said reversed feedback is obtained by means of a resistance in the cathode grid circuit of each stage, and said voltage is applied to vary the grid bias of said stages.

3'. Receiving apparatus for the reception of trains of modulated electrical pulses comprising an amplifier having a negative feedback path in each stage and a feedback path connecting the output to the input of the amplifier, said stages operating as inverted amplifier stages, with the control grids thereof earthed and the inputs applied to the cathodes, but with the control bias voltage applied to said grids, and said feedback path connecting the output to the input of the amplifier providing a broad passband of frequencies and having a delay equal to the repetition period of the received pulses, and means for producing a voltage responsive to the depth of modulation of the received pulses, said voltage being applied to vary automatically the amount of the negative feedback in said amplifier.

4. Receiving apparatus for reception of frequency modulated waves comprising an amplifier having a variable width band pass characteristic, means to vary the band width of said amplifier, means for producing a voltage responsive to the depth of frequency modulation of the received waves comprising a slot filter and rectifier, said slot filter comprising a, bridged-T network arranged to reject the unmodulated carrier frequency, a further network to which the output of said network is fed, said further network being tuned approximately to the side bands of the received waves, the output of said further network being fed to said rectifier, a resistance in the output of said rectifier across which a con-' trol voltage is obtained, and means to apply said control voltage to vary the band width of said amplifier.

5. Receiving apparatus, as claimed in claim 4, wherein said bridged-T network feeds into two tuned circuits. one tuned slightly above and the other slightly below the unmodulated carrier frequency, said tuned circuits feeding respective rectifiers, the outputs of which are fed to a common resistance from which the desired control voltage is obtained.

6. Receiving apparatus for the reception of time modulated electrical pulses comprising a variable width band pass filter, means for producing an intermediate frequency voltage responsive to the depth of modulation, means for applying said voltage to said filter to vary the Width of its band pass characteristic to control the frequency band width of the receiver, wherein said means for producing a voltage responsive to the depth of modulation of the received pulses comprises a slot filter of the comb-type having multiple response frequency bands and a rectifier, said slot filter comprising two electrical paths in parallel, having a differential delay equal to the unmodulated pulse repetition frequency and having equal attenuations, means for feeding the outputs of said paths to said rectifier, and a resistance across which the desired control voltage is obtained and into which the rectifier feeds.

7. Receiving apparatus for the reception of electromagnetic radio frequenc waves modulated in such manner as to vary the transmitted frequency band width comprising means for producing an intermediate frequency voltage varying according to depth of frequency modulation, means for converting the intermediate frequency voltage into audio frequency voltage, a filter between said means for producing and means for converting, and means for applying part of the intermediate frequency voltage to control the frequency band width of said filter, the last mentioned means comprises a second filter connected between the means for converting and said filter.

CHARLES WILLIAM EARP.

REFERENCES CITED The following referen ces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 21,598 Walsh Oct. 8, 1940 1,926,097 Hansell Sept. 12, 1933 2,002,216 Bode May 21, 1935 2,017,523 Beers Oct. 15, 1935 2,051,364 Braden Aug. 18, 1936 2,054,412 Farrington Sept. 15, 1936 2,054,892 Braden Sept. 22, 1936 2,083,232 Koch June 8, 1937 2,120,998 Barber June 21, 1938 2,152,515 Wheeler Mar. 28, 1939 2,183,980 Wheeler Dec. 19, 1939 2,190,243 Robinson Feb. 13, 1940 2,261,374 Koch Nov. 4, 1941 2,282,973 Koch May 12, 1942 2,282,974 Koch May 12, 1942 2,316,017 Peterson Apr, 6, 1943 

